1. Field of the Invention
The present invention relates to diazo photoreproduction, and more particularly, to coupling compounds for use in diazo photoreproduction.
2. Description of the Prior Art
In semi-dry or two component diazotype photoreproduction, one or more coupling components are included as ingredients of a sensitizing composition. The sensitizing composition also includes a diazo sensitizer which decomposes when subjected to actinic radiation. Accordingly, after exposure, coupling between the coupling component or components and the diazo sensitizer can only occur in those areas where decomposition was not complete. Development is effected by subjecting the exposed composition to alkaline atmosphere, e.g., by bringing it into contact with ammonia vapors, to neutralize the acidic inhibitor with the concomitant formation of dyestuff in the non-exposed areas due to coupling of the residual diazo compound and the coupling components.
Although the color of the azo dye image which is obtained in any given instance depends primarily on the coupling components and the diazonium compounds which are employed, coupling components are typically described as being couplers of a given color, with the color being the color of the dye which is usually obtained when the particular coupler in question couples with a diazonium compound. For example, couplers such as monohydric phenols, catechols, catechol derivatives, resorcinols, resorcinol derivatives, diketones, acetoacetic acid derivatives, acetonitriles, cyanoacetamides and the like, usually result in yellow, orange, sepia, brown, red or maroon azo dyes. Thus, couplers from such classes of materials are conveniently referred to as yellow, orange, sepia, brown, red, or maroon couplers. On the other hand, couplers such as hydroxy naphthoic acid derivatives, dioxynaphthalene derivatives, pyronones, hydroxypyridones, and the like, usually result in blue or violet azo dyes, and thus are conveniently referred to as blue or violet couplers.
One group of highly useful coupling components are the yellow couplers, since the dyes obtained from these couplers usually have actinic absorption characteristics which permit their use as the sole coupler in a diazo composition which is employed to prepare diazotype "masters" or intermediates, and since couplers from this group can often be employed as shading components when used in conjunction with another coupler or couplers. As indicated above, compounds containing active methylene groups, compounds such as acetonitriles, derivatives of acetonitriles, and the like, have been employed as yellow couplers in diazo compositions, (cf., for example, U.S. Pat. Nos. 1,989,065; 2,531,004, 2,537,001; and 2,537,106), yet a number of these active methylene types of couplers have exhibited a tendency, when employed in two-component diazo compositions, to precouple with the diazonium compound which is present in said compositions during storage even in the presence of the stabilizers which are usually employed. This tendency to precouple prior to exposure and development has limited the use of these materials somewhat, since even a slight amount of precoupling can result in the formation of an azo dye in those areas of the diazotype material which are the background or "cleared" areas of the diazotype print. In addition to this tendency to precouple, a number of these prior-art, active-methylene types of couplers also result, upon coupling, in dyes which have an undesirable reddish hue and/or which have a tendency to fade upon subsequent exposure to light.
It should be apparent from the above, that, in addition to obtaining a single-color azo dye image, one should be able to obtain a mixture of azo dyes (and thus a mixture of colors) by including more than one coupling component or more than one diazonium compound in the light-sensitive diazo composition. Thus, by a proper choice of coupling components and/or diazonium compounds, one should be able to obtain a wide variety of colors in the resulting azo dyes, including black. However, the achievement of a uniform color over a wide range of image densities from a diazo composition containing more than one coupling component has proven difficult to obtain in actual practice. In order to obtain a uniform color over a wide range of image densities, it is essential to carefully match the coupling activity of the various coupling components with the diazonium compound or compounds which are employed, and that the combined absorptions of the azo dyes produced from the various couplers cover the entire visible spectrum. It is also essential that none of the azo dyes produced from the various coupling components be subject to a "color-shift" or change of shade due to a change in pH, for example, after ammonia has evaporated from the film. Otherwise, the resulting dye image of the diazotype material may shift from the neutral point.
Thus although resorcinol sulfides, resorcinol sulfoxides (for example, see U.S. Pat. No. 2,717,832 and 3,785,826) and diresorcinols (for example, see U.S. Pat. No. 2,542,560 and 2,542,566) are several classes of yellow azo coupling components which exhibit excellent resistance to subsequent fading to light, these particular classes of yellow coupling components unfortunately exhibit a severe tendency to color-shift when incorporated into black-line formulations. Similarly, AON's such as 1-hydroxy-2-naphthoic acid piperidide also exhibit a severe tendency to color-shift when incorporated into black-line formulations. In addition, these compounds are ultraviolet absorbers and readily discolor with light.
Attempts have also been made to employ alkyl substituted resorcinols (e.g., 2-alkyl resorcinols) and dialkyl substituted resorcinols (e.g., 2,4-dialkyl resorcinols) in blackline formulations. Although the diazotype materials resulting from such formulations exhibit little or no tendency to color-shift, these particular classes of resorcinol coupling components have a relatively slow rate of coupling and are readily oxidized to colored oxidation products.
Phenolic couplers such as o-biphenol, 1,2-bis-o-hydroxy phenyl cyclopropane, meta-cresyl glutaric acid, and the hydroxyethyl ether of catechol are commercially used to give lemon-yellow colors. However, their coupling activity leaves much to be desired. Although neutral blacks are obtained in the high density areas, the blue (violet-coupling) coupler predominates in the lower density areas and a two-toned print is obtained.
There have also been attempts to produce modified diresorcinols for use as coupling components. For example, U.S. Pat. Nos. 3,573,052 and 3,591,381 state that the following class of hindered phenols as stabilizers for use with blue and yellow couplers: ##STR1## wherein each of R.sub.6 to R.sub.9 is an alkyl radical, and R.sub.6 and R.sub.7 are preferably tertiary alkyl radicals. The '052 patent further discloses the following compounds: ##STR2## wherein R.sub.7 -R.sub.9 are alkyl radicals having between one and eighteen carbon atoms.
U.S. Pat. Nos. 3,619,191 and 3,857,896 disclose the synthesis and use of the following diresorcyl sulfide and sulfoxide compounds as orange or sepia couplers in a diazo process: ##STR3## wherein R.sup.4 is hydrogen, an alkyl group containing from one to six carbon atoms, an unsubstituted aryl group or halo-substituted aryl group, an aralkyl group, an alkaryl group, an alkoxy group having from one to six carbon atoms, an aryloxy group, an aralkoxy group, or a halogen; and Z is a hydrogen or halogen atom.
U.S. Pat. No. 3,881,931 discloses a yellow coupler of the following general formula: ##STR4## wherein R.sub.8 ad R.sub.10 each represent an alkyl group having one to four carbon atoms, and R.sub.9 and R.sub.11 each represents either a hydrogen atom or a halogen atom.
However, in spite of the activity in this area, a need continues to exist for coupling components which give lemon-yellow colors with rapid developing diazonium compounds, are resistant to light oxidation, are resistant to oxidation to colored products, are not UV absorbers, couple very rapidly, and do not exhibit pH ammonia shift of the dyes once formed.